Foil strain gages of the type mentioned above are known from British Patent 728,606 (Eisler), published on Apr. 20, 1955. According to this publication, a meandering measuring resistor grid is formed of a very thin rolled out metal foil by a photolithographic etching technique, whereby the ends of the measuring resistor grid are formed as contact area. Thereafter, an electrically insulating epoxy resin backing is applied to the metal foil. Since the epoxy resin backing is rather brittle, it is necessary to handle such foil strain gages with the utmost care and precaution during manufacturing, during handling, and during application of the foil strain gage to an element in which forces are to be measured in order to avoid breaking the strain gage. The same basically applies to load cells comprising such foil strain gages. The same considerations apply to strain gages with backings made of phenolic resin. As a result, conventional strain gages with epoxy resin backings and with phenolic resin backings have not been used in tension analysis applications.
Suggestions have been made to reinforce phenolic and epoxy resin backings with glass fiber fleeces. However, foil strain gages having such backings are still rather brittle and break easily, especially during cutting and during handling. Another drawback of glass fiber reinforced backings is seen in the fact that the glass fiber fleeces themselves have faults so that the waste is quite substantial during manufacture and application of such strain gages in cells and transducers.
Foil strain gages with polyimide or polyamide backings are known to have rather robust characteristics and strain gages of this type have been used in tension analysis applications. However, strain gages with polyimide or polyamide backings are not suitable for precision measurements, due to their strong creeping characteristics.
Another problem with conventional foil strain gages resides in the fact that the backing and/or the cover or encapsulation changes its characteristic in response to the relative humidity either by swelling or by shrinking. Both, swelling and shrinking are undesirable because they cause a direct change in the resistance of the measuring grid, whereby a humidity dependent null signal is unavoidable. Still another drawback of conventional foil strain gages is the fact that the creeping characteristic is temperature dependent. Such dependency of the creeping on the temperature is not easily controlled with economically feasible compensating measures, whereby these strain gages are useful only for predetermined temperature ranges. Swelling and/or shrinking have an even larger adverse effect on the measured result when the backing of the strain gage and the cover of the strain gage are exposed to conditions causing swelling and/or shrinking. German Patent (DE-PS) 4,011,314, corresponding to U.S. Pat. No. 5,192,938 (Ort), issued on Mar. 9, 1993, discloses foil strain gages with a backing of polyphenylenesulfide. These foil strain gages have excellent robust mechanical characteristics with optimal creeping properties and a very small tendency to swell. PPS provides an excellent seal against moisture penetration. Furthermore, foil strain gages with polyphenylenesulfide backings can be manufactured economically and these strain gages are free of hysteresis faults as well as faults that can be caused by the environment so that highly precise measuring load cells can be produced. However, there is still room for improvement regarding the bonding of the measuring resistor grid structure to the polyphenylenesulfide backing and to the covering. These improvements are especially desirable with regard to the peel strength between the measuring resistor grid structure and the backing, and with regard to the thermal resistance, especially at very high temperatures.